Diesel fuels and/or biodiesel fuels typically contain wax, and when subjected to low temperatures, these fuels often undergo wax crystallization, gelling and/or viscosity increase. This reduces the ability of the fuel to flow and creates filter plugging which adversely affects the operability of vehicles using these fuels. Flow improvers have been used to modify the wax structure as it builds during cooling. These additives are typically used to keep the wax crystals small so that they can pass through fuel filters. Also, pour point dispersants are sometimes used in diesel fuel to ensure that it can be pumped at low temperatures.
Due to environmental concerns and the decline of known petroleum reserves with subsequent price increases of petroleum, biodiesel fuels are becoming a focus of intense research and development efforts. Biodiesel fuels typically comprise fatty acid esters, prepared for example by transesterifying triglycerides with lower alcohols, e.g. methanol or ethanol. A typical biodiesel fuel is the fatty acid ester of a natural oil (i.e. rapeseed oil or of soybean oil, as non-limiting examples). One of the major problems associated with the use of biodiesel is its poor cold flow properties resulting from crystallization of saturated fatty compounds in cold conditions, as indicated by its relatively high cloud points (CP) and pour points (PP). A 20° C. reduction in cold filter plugging point is necessary for some biodiesel fuels to find utility in colder climates such as those of North America and Europe in winter.
Several efforts to mitigate the low-temperature problems of biodiesel have been investigated over the past several years. Many popular approaches have included blending biodiesel with conventional diesel fuel, winterization, and use of synthetic additives. Also, studies have been performed to show the diversification in the feedstock and genetic modification of the feedstock, aimed to provide a reduction in the saturated content of the fatty acid methyl esters (FAME) in biodiesel as well as modification of FAME composition/profile of the fuels. While there have been efforts to create additives that may reduce the PP and cold filter plugging point (CFPP) of fuels, many are not cost effective. Also, increasing the unsaturated content of biodiesel may improve its cold flow properties, but also leads to the alteration of the oxidative stability of the fuel. The overall thermal behavior of biodiesel is affected by the relative concentration of its saturated and unsaturated FAME components. The cold flow issue is primarily a multifaceted problem of crystallization (of saturated FAMEs) in solution (unsaturated FAMEs) which can be approached from several angles.
Several approaches have been utilized to lower the onset temperature of crystallization of biodiesel, targeting particularly the saturated FAMEs such as methyl palmitate (MeP) and methyl stearate (MeS), which influence most its flow behavior at low temperature. The most popular approach is the use of crystallization depressant additives.
Saturated triacylglycerols (TAGs) and dimers of TAGs, particularly those having two double bonds at the sn-1 and sn-3 positions, have been found to be effective in suppressing the crystallization of FAMEs. One structured triacylglycerol dimer, (E)-1-(1-(oleoyloxy)-3-(stearoyloxy)propan-2-yl) 18-(1-(oleoyloxy)-3-(stearoyloxy) propan-2-yl) octadec-9-enedioate (Compound D), which can be produced from self-metathesis of natural oils, is one compound that has been found to significantly reduce the crystallization temperature of biodiesel.